hotspot-manager: use mechanicd exclusively to switch WiFi mode
In Focal, we no longer patch wpa_supplicant just to add single custom method. In its place, Alfred's mechanicd has a functionality to switch WiFi mode, used already by Aethercast.
Thus, move lomiri-indicator-network to also use this new mechanism. This allows us to drop MTK-specific code.
And while we're at it, also fix the incorrect usage of std::unique_ptr
.
Fixes: #104 (closed)